A relatively new technology uses semiconductor powders to carry out photocatalytic processes. Irradiation of the semiconductor with light of energy equal to or greater than the band gap results in the creation of holes in the semiconductor valence band and electrons in the semiconductor conduction band followed by trapping of the separated charges in shallow traps at the semiconductor particles surface. The injection of these electrons and holes into the region surrounding the semiconductor particles surface causes a chemical modification of substances within the region. Such technology has been used in at least the following photocatalytic processes: the photo-Kolbe reaction in which acetic acid is decomposed to methane and carbon dioxide; the photosynthesis of amino acids from methane-ammonia-water mixtures; the decomposition of adipic acid to carbon dioxide, butane, and valeric acid; the production of hydrogen from several aliphatic and aromatic compounds (including fossil fuels) with water; the degradation of chlorinated hydrocarbons to carbon dioxide and hydrochloric acid; the oxidation of cyanide; the sterilization against aqueous microbial cells; and the deposition of metals from their aqueous ions. Some of the exemplary literature describing experiments utilizing such technology are: "Photocatalytic Reactions of Hydrocarbons and Fossil Fuels with Water. Hydrogen Production and Oxidation", by K. Hashimoto, T. Kawai, and T. Sakata, J. Phys. Chem., Vol. 88, No. 18, pp. 4083-4088, 1984; "Solar Photoassisted Catalytic Decomposition of the Chlorinated Hydrocarbons Trichloroethylene and Trichloromethane", by S. Ahmed and D. Ollis, Solar Energy, Vol. 32, No. 5, pp. 597-601, 1984; "Heterogeneous Photocatalytic Decomposition of Benzoic Acid and Adipic Acid on Platinized TiO.sub.2 Powder. The Photo-Kolbe Decarboxylative Route to the Breakdown of the Benzene Ring and to the Production of Butane", by I. Izumi, F. F. Fan, and A. J. Bard, J. Phys. Chem., Vol. 85, No. 3, pp. 218-223, 1981; "Heterogeneous Photocatalytic Oxidation of Aromatic Compounds on TiO.sub.2 ", by M. Fujihira, Y. Satoh and T. Osa, Nature, Vol. 293, pp. 206-208, 1981; "Powder Layer Photoelectrochemical Structure", by R. E. Hetrick, J. App. Phys., Vol. 58, No. 3, pp. 1397-1399, 1985; "Solar Assisted Oxidation of Toxic Cyanide", Langley Research Center, Hampton, Virginia, NASA Tech Briefs, p. 106, Spring 1985; "Photocatalytic Decomposition of Water and Acetic Acid Using a Powder-Layer Photoelectrochemical Structure", by R. E. Hetrick, App. Phys. Comm., Vol. 5, No. 3, pp. 177-187, 1985; "Photoelectrochemical Sterilization of Microbial Cells By Semiconductor Powders", by T. Matsunaga. R. Tomoda. T. Nakajima, and H. Wake, FEMS Microbiology Letters, Vol. 29, pp. 211-214, 1985"Photocatalytic Deposition of Metal Ions Onto tiO.sub.2 Powder", by K. Tanaka, K. Harada, and S. Murata, Solar energy, Vol. 36, No. 2, pp. 159-161, 1986.
It is believed that all present systems utilizing the technology maintain the chemical compounds to be modified in a gas mixture, gas solution, a gas/liquid mixture, a liquid, or another fluid for the most effective contact between the chemical compound and the semiconductor powder. In most of the systems, the semiconductor powder itself is also suspended and mixed in the gas or liquid and is maintained in such suspended and mixed condition by bubbling a gas through a liquid, constantly stirring the fluid with a magnetic stirrer, for example, or continuously circulating the fluid with a pump. A problem with suspending and mixing the semiconductor powder in a gas or liquid is that some means must be utilized to maintain the semiconductor powder in a suspended and mixed condition and that the semiconductor powder must be segregated from the modified chemical compound, especially if the semiconductor powder is to be reused. In another system, a porous layer of semiconductor powder is placed on a metalized substrate by dispersing the powder in a polymeric binder dissolved in an organic solvent, spinning the mixture onto the metalized substrate, and burning off the binder. Two major drawbacks of placing the semiconductor powder on a metalized substrate are that the semiconductor powder is prone to becoming disengaged and displaced from the substrate because of the relatively very weak forces attracting the powder to the substrate and that only a thin sheet of gas or liquid can come into intimate contact with the semiconductor powder in order to modify the chemical compound. It also appears that the known systems carry out photocatalytic processes on a batch or intermittent basis for modifying relatively small amounts of chemical substances. Consequently, there is a need for a system which will maintain the semiconductor powder in a predetermined position while at the same time permitting relatively large volumes of the gas or liquid containing the chemical compound to be modified to pass through and come into intimate contact with the semiconductor powder in a continuous or plug flow.